I'm surprised that no one has yet mentioned monitoring.
The alternative to building classic object-oriented software guided by tests is to develop microservices with extensive real-time monitoring and alerting. If something's broken, the service will go down, or a metric will spike up/down, and the developer who owns the microservice needs to fix it. This approach is sometimes called programmer anarchy and requires a high level of maturity across the whole team.
That's basically what I'm doing.
Didn't consider this as an alternative to unit tests until now.
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